Answer
$-1$
Work Step by Step
$\lim _{x\rightarrow b}\dfrac {\left( x-b\right) ^{50}-x+b}{x-b}=\dfrac {\left( x-b\right) ^{50}-\left( x-b\right) }{x-b}=\dfrac {\left( x-b\right) \left( \left( x-b\right) ^{49}-1\right) }{\left( x-b\right) }=\left( x-b\right) ^{49}-1=0^{49}-1=-1$
